Field-cycling NMR Relaxometry: Instrumentation, Model Theories and Applications
Chapter 9: Nuclear Magnetic Relaxtion Dispersion of Water–Protein Systems
Published:11 Oct 2018
Applications of 1H MRD to aqueous and hydrated protein systems are presented, including solutions, crosslinked gels and hydrated solids. The water–proton relaxation is coupled to the protein dynamics by proton and water-molecule chemical exchange with protein sites that modulates both inter- and intramolecular 1H dipolar couplings. The nearly Lorentzian shape of the 1H2O relaxation dispersion for protein solutions reports the rotational correlation time of the protein, which may be strongly affected by aggregation effects. Although the relaxation dispersion reflects the distribution of aggregate sizes, several factors make it difficult to extract the size distribution. For protein systems where the rotational motion is quenched, the relaxation dispersion is a power law in the Larmor frequency, which may reflect the underlying power law for the relaxation dispersion of the rotationally immobilized protein protons and also the distribution of water and proton exchange times that may additionally interrupt the bound-state inter- and intramolecular dipolar couplings. At high Larmor frequencies, 1H2O relaxation is dominated by fast rotational and translational dynamics of water in the protein interface, where both a distribution of weak binding events and restricted translational diffusion may contribute to the total logarithmic relaxation dispersion.